1. Field of the Invention
The present invention relates to a method for fabricating a novel electrode for lithium secondary battery.
2. Related Art
In a lithium secondary battery having been actively researched and developed recently, battery characteristics such as charge/discharge voltages, charge/discharge cycle life characteristics and storage characteristic are greatly influenced by an electrode used. Therefore, battery characteristics are enhanced by improving an active material used for an electrode.
Although it is possible to constitute a battery having high energy densities per weight and volume by using lithium metal as a negative active material, a problem occurs that lithium is deposited like dendrite to cause an internal short-circuiting.
On the other hand, a lithium secondary battery is reported which uses any one of aluminum, silicon and tin which are electrochemically alloyed with lithium during charge (Solid State Ionics, 113-115, p.57(1998)). Among the above materials, silicon is particularly prospective as negative electrode for a battery with a high capacity, having a large theoretical capacity. For this reason, various secondary batteries respectively using silicon for the negative electrode are proposed (Japanese Patent Laid Open No. Hei 10-255768). In the case of the negative alloy electrode of this type, however, a sufficient cycle characteristic is not obtained because the alloy itself, which is an active material, is pulverized due to charge/discharge and thereby, the current-collecting characteristic is deteriorated.
In order to overcome these problems, proposed is an electrode for lithium secondary battery obtained by forming a microcrystalline silicon thin film or an amorphous silicon thin film on a current collector by a thin-film forming method such as CVD method or sputtering method, which electrode shows good charge/discharge cycle characteristics (International Patent Laid Open WO01/31720A1 etc.).
It is an object of the present invention to provide a method for fabricating an electrode for lithium secondary battery using an active thin film such as a silicon thin film, having high charge/discharge capacities, and being superior in charge/discharge cycle characteristic.
The first aspect of the present invention is a method for fabricating a electrode for lithium secondary battery to be formed by depositing an active thin film capable of lithium storage and release, on a metallic foil used as a current collector, in which the surface of the metallic foil is roughened by wet-etching, and the active thin film is deposited on the roughened surface.
The second aspect of the present invention is a method for fabricating an electrode for lithium secondary battery to be formed by depositing an active thin film capable of lithium storage and release, on a metallic foil used as a current collector, in which the surface of the metallic foil is roughened by spraying particles, so called xe2x80x9csand blastingxe2x80x9d, to make collision with the surface of the metallic foil, and the active thin film is deposited on the roughened surface.
An active thin film of the present invention is formed by depositing the film on a metallic foil. A method for supplying a material from a gaseous phase and depositing an active thin film is preferably used as a method for forming the active thin film. This type of method includes sputtering, CVD, vacuum evaporation, and thermal spraying processes. Moreover, a method for forming an active thin film from a liquid phase includes electrolytic plating and electroless plating processes.
An active thin film of the present invention is a thin film made of an active material for storing and releasing lithium. As an active thin film, an active thin film which stores lithium by being alloyed with lithium is preferably used. As a material for the above active thin film, silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, and indium and the like are exemplified.
An active material in which silicon or germanium is main component is preferable from the viewpoint that it is easy to form a thin film through the thin-film forming method from gaseous phase. Moreover, an active material in which silicon is main component is particularly preferable from the viewpoint of high charge/discharge capacities.
Moreover, it is preferable that an active thin film is an amorphous thin film or a microcrystalline thin film. Therefore, an amorphous silicon thin film or a microcrystalline silicon thin film is preferably used as an active thin film. The amorphous silicon thin film is a thin film in which a peak around 520 cmxe2x88x921 corresponding to a crystal region is not substantially detected through the Raman spectroscopic analysis, and the microcrystalline silicon thin film is a thin film in which both a peak around 520 cmxe2x88x921 corresponding to a microcrystalline region and a peak around 480 cmxe2x88x921 corresponding to an amorphous region are substantially detected through the Raman spectroscopic analysis. Further, it is preferable to use an amorphous germanium thin film, a microcrystalline germanium thin film, an amorphous silicon-germanium alloy thin film, and a microcrystalline silicon-germanium alloy thin film.
A metallic foil used as a current collector in the present invention is not restricted as far as the foil can be used as a current collector of an electrode for lithium secondary battery. However, it is preferable that the metallic foil is made of a metal not to be alloyed with lithium. As such a metallic foil, for example, metallic foils made of copper, iron, nickel, tantalum, molybdenum or tungsten, or an alloy containing at least one of these metals are used.
In the first aspect of the present invention, the surface of a metallic foil is roughened by wet-etching and then an active thin film is deposited on the roughened surface. By wet-etching the surface of the metallic foil, the surface roughness Ra of the metallic foil surface is preferably set to 100 nm (0.1 xcexcm) or more, further preferably set to 150 nm (0.15 xcexcm) or more, or still further preferably set to 200 nm (0.2 xcexcm) or more. The surface roughness Ra is specified in Japanese Industrial Standards (JIS B 0601-1994) and it can be measured by a surface roughness meter or a scanning probe microscope (SPM).
An etchant used for wet-etching is not restricted as far as can etch the surface of an objective metallic foil. As such an etchant, a hydrochloric acid based etchant may be used.
In the second aspect of the present invention, the surface of a metallic foil is roughened by spraying particles on the surface of the metallic foil to collide the particles with the surface. As a particle to be sprayed on the surface of the metallic foil, generally used is a particle made of a material harder than the metallic foil. As such a particle, a particle made of alumina, silicon carbide, glass, steel, stainless steel, zinc, or copper is used.
It is possible to control the degree of roughening of the surface of a metallic foil in accordance with the type, size, quantity, and spraying pressure of particles to be sprayed on the surface. Moreover, in general, particles are made to collide with the surface of a metallic foil by feeding a metallic foil so as to pass through an area to which particles are sprayed. It is possible to control the degree of roughening also by the feed rate of the metallic foil.
It is possible to use a particle having a size at a grade of #200 to #2,000, that is, having a maximum particle diameter of 10 to 150 xcexcm as a size of a particle.
In the second aspect of the present invention, it is preferable that the surface of a metallic foil is roughened so that the surface roughness Ra becomes 0.1 xcexcm or more. More preferably, it is roughened so that the surface roughness Ra becomes 0.15 xcexcm or more and still further preferably, it becomes 0.2 xcexcm or more. The surface roughness Ra is specified in Japanese Industrial Standards (JIS B 0601-1994) and it can be measured by a surface roughness meter or a scanning probe microscope (SPM).
Because a surface oxide film may be formed on the surface of a metallic foil, it may be preferable to remove the surface oxide film before depositing an active thin film. In the second aspect of the present invention, because the surface of a metallic foil is roughened by making particles collide with the surface, it is possible to remove a surface oxide film from the surface of the metallic foil when roughening. Moreover, when the surface of a metallic foil is rustproofed and thereby, a rustproofing layer is present on the surface, it is possible to remove the rustproofing layer when roughening.
In the present invention, an interlayer may be provided on the roughened surface of a metallic foil and an active thin film may be provided on the interlayer. We have found that the diffusion of the current collector component in the active thin film to form a solid solution improves the adhesion between the current collector and the active thin film as well as the charge/discharge cycle characteristic. From this viewpoint, when a metallic foil does not contain a component diffusing in an active thin film, it is preferable to provide an interlayer containing a component which diffuses in an active thin film to form a solid solution. When a silicon thin film is used as an active thin film, and a nickel foil is used as a metallic foil, it is preferable to provide an interlayer made of a copper layer on a nickel foil and provide a silicon thin film on the interlayer, because nickel (Ni) does not easily form a solid solution with silicon (Si). Copper (Cu) is a component that diffuses in a silicon thin film and easily forms a solid solution.
According to the present invention, an active thin film is formed by depositing the film on the surface of a metallic foil roughened through wet-etching or collision with particles. Because the surface of the metallic foil is roughened, the contact area between the active thin film and the surface of the metallic foil increases, thereby to improve the adhesion of the active thin film to the metallic foil. Therefore, even if the active thin film is expanded and shrinked due to a charge/discharge reaction, the active thin film is prevented from delaminating from the current collector by the stress.
Moreover, we have found that when forming an active thin film on the surface of a roughened metallic foil, irreguralities corresponding to irreguralities on the surface of the metallic foil are formed on the surface of the active thin film. Furthermore, we have found that in an active thin film with irreguralities on its surface, gaps are formed in a manner to extend in its thickness direction from valleys of the irreguralities of the thin film surface when the thin film is expanded and shrinked due to a charge/discharge reaction, and that these gaps serve to lessen a stress engendered due to expansion and shrinkage of the thin film on charge and discharge. As a result, an excellent charge/discharge cycle characteristic is obtained.
Furthermore, according to the second aspect of the present invention, the surface of a metallic foil is roughened by spraying particles to collide the particles with the surface. Therefore, it is possible to control at will the degree of roughening the surface of a metallic foil by adjusting the type, size, quantity, or spraying pressure of the particles. Furthermore, according to the second aspect of the present invention, various materials can be used for a metallic foil. Furthermore, because a roughening step is a drying step, it is possible to roughen the surface of a metallic foil without complicating the fabrication process.